I have a PHP class that creates a PNG image on the fly and sends it to browser. PHP manual says that I need to make sure that imagedestroy function is called at end to release the memory. Now, if I weren't using a class, I would have some code like this:

Can you add a link to anything in the php manual or otherwise that supports your test? That would be of great benefit to us all.
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Noah GoodrichOct 25 '08 at 19:07

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If you have a working setup, just copy/paste this examples I gave and try yourself. It it was explained in PHP manual or otherwise I wouldn't ask here at SO.
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Milan BabuškovOct 25 '08 at 19:16

This behavior is not "by design" so it may change in future versions. If no output is send to the browser the stop button has no effect. "echo" is a function that can cause a fatal error (broken pipe). Thanks for your research but in the GD img case everybody should use the destructor method.
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Bob FangerOct 26 '08 at 15:39

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That's documented in the PHP Manual. Both, the registered shutdown function and destructors get called when the script ends. However, if the connection state changes to ABORTED (php has sensed user pressed the browsers stop button) and ignore_user_abort is not set to true, the registered shutdown function will be called not when the script ends, but when this was sensed. See Shutdown/Destructor and connection handling.
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hakreJun 3 '11 at 12:55

Based on the principle that you should finish what you start, I'd say the destructor is the correct place for the free call.

The destructor will be called when the object is disposed of, whereas a shutdown function will not be called until script execution finishes. As noted by Wolfie, these won't necessarily happen if you forcibly halt the server or the script, but at that time, the memory allocated by PHP will be freed anyway.

Also noted by Wolfie, PHP will free up script resources when the script closes, so if you're only instantiating one of these objects, then you probably wouldn't notice a massive difference. However, if you later do end up instantiating these things, or do so in a loop, then you probably don't want to have to worry about a sudden spike in memory usage, so for the sake of future sanity, I return to my original recommendation; put it in the destructor.

I think one big thing that you have missed is that all the memory PHP has allocated during script execution is freed once the script terminates. Even if the user presses the stop-button, PHP processes the script until it is finished, gives it back to the HTTP daemon to be served to the visitor (or not, depending on how clever the daemon is).

So, explicitly freeing up memory at the end of script execution is a bit redundant. Some might argue that it would be a good thing to do, but it's still redundant.

But, on the topic of class destructors, they are called whenever the object is destroyed, either explicitly by unset() or at script completion/termination.

The recommendation of the developer explicitly freeing up the memory used in image manipulation is sure just to make absolutely sure not to have a memory leak, as bitmaps can be straining on the memory side of things (height * width * bit depth * 3 (+ 1 if you have an alpha channel))

AFAIU, memory is not allocated by PHP because it invokes GD to create images and GD does not have garbage collection.
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Milan BabuškovOct 25 '08 at 18:33

I already read PHP manual, but it does not explicitly say whether destructor is called if script is terminated by web server.
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Milan BabuškovOct 25 '08 at 18:34

also AFAIK, GD is just a library that is dynamically linked with PHP. This would imply that while GD knows how to do all the stuff, it's still PHP's memory and thus PHP's garbage collection.
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Henrik PaulOct 25 '08 at 18:52

@wolfie: no. I just tested and it leaks memory. Since PHP is Apache module, it gets returned to OS when Apache is restarted, which I don't do that often (any hosting company tries not to restart web server at all)
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Milan BabuškovOct 25 '08 at 18:53

The situation may be different depending on whether it's a module or a cgi/fast-cgi setup.
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HansDec 21 '12 at 18:38